Grain oriented silicon steel having about 2 to 4.5% silicon requires careful processing to control the final grain size, orientation and coating conditions which provide good uniform magnetic properties.
The hot rolling process for oriented silicon steel requires a slab temperature which dissolves the inhibitors which later precipitate during hot rolling. As taught in U.S. Pat. No. 2,599,340, the slab temperatures are typically 1260.degree. to 1400.degree. C. to dissolve the grain growth inhibitors.
U.S. Pat. No. 3,764,406 recognized a difference in grain growth depending on the casting process. Continuous cast slabs have excessive grain size if processed like the ingots. A prerolling process was discovered which subjected the slab to a reduction of 5-50% at a temperature below 1250.degree. C. to limit the grain growth during the completion of the hot rolling process. The prerolled slab was then heated to 1260.degree. to 1400.degree. C. to dissolve the inhibitors and prepare the slab for final hot rolling.
U.S. Pat. No. 4,330,348 recofnized some of the slab heating problems in a pusher-type furnace. The continuous cast slab had the lowest slab temperature portion (in contact with the furnace skids) carefully monitored to control secondary recrystallization.
U.S. Pat. No. 4,088,513 requires a walking-beam type furnace and properly spaced slabe to adjust for the slag conditions and improve the atmosphere circulation beneath the slab. The slag was recognized as causing yield loss and surface damage on the slab when pushed across the skids or furnace bottom.
Prior solutions to control grain size and prepare the slab for hot rolling have not addressed the internal oxidation process which results in a silicon-free iron layer with the appearance of streaks and surface scale conditions existing on the slab as it exits the furnace. Pusher-type heating furnaces have a more significant problem and causes deterioration and nonuniformity of the magnetic properties in the final strip. The streaks also cause breakage during cold rolling.
Accordingly, there remains a need for a process to eliminate the streaks which are observed after the glass film formation but are caused by the slab furnace heating conditions. Furthermore, there remains a need for a process which can be adapted to existing hot rolling equipment for silicon steel which does not require considerable equipment change or significant reduction in productivity.